Brake pressure..

[MrK]

Ever since I installed higher compression pistons, I've been having some trouble with my brake pressure. Unless I rev the engine to about 2 or 3k, my pedal is stiff and unresponsive. It's as if I need to rev the engine to allow the MC to collect enough vaccuum to work properly.

The result is that at low engine speeds, I only have a limited ability to brake, and while this isn't outright dangerous (since at low rpms I'm probably not going very fast), I'd like to know the root of the problem and restore normal brake pressure.

Any ideas?

[rs4pro3]

the root is your higher CR ratio pistons, throwing off the vacume of the engine, that the brake booster uses to supply your braking power.
You probally need to change the vacume check valve out to one that opens at a different pressure.

[MrK]

Ok great, that's pretty much what I thought. Although I'd really appreciate a little more detail, i.e. where is the vaccuum check valve, and what one to get instead of my existing one.

Thx

[christophbmw]

the check valve is located "in-line" on the vacum hose between your brake booster and intake manifold......very hard to miss. i do not know where you can get a different check valve, maybe someone here knows.

[DaN]

I would check if the booster holds vacuum or not.

I'm not sure that a bump in compression would make that much of a difference. How much vacuum does the engine create?

[NC325iC]

what would happen if you removed that valve completely? would that mean the brakes would be on all the time or would it mean youd get max braking power all the time

[MrK]

Well I checked the booster itself, no vaccuum leaks anywhere, it seems to work just fine. And I had perfectly normal brake pedal feel before I screwed around with the engine. So the booster itself can be ruled out.

[DaN]

what would happen if you removed that valve completely? would that mean the brakes would be on all the time or would it mean youd get max braking power all the time

You would lose brake boost under wide open throttle, and you would lose any left over boost with the engine off.

The vacuum doesn't apply the brakes for you, it assists.

[arsevader]

the root is your higher CR ratio pistons, throwing off the vacume of the engine, that the brake booster uses to supply your braking power.
You probally need to change the vacume check valve out to one that opens at a different pressure.

The check valve is just there to provide braking assist in case of engine failure.

It doesn't open at any particular pressure it's just a one way valve. You can blow through it with your mouth one way and not the other.

This is to prevent pressure loss in the booster in case the engine ceases to provide vacum (engine stops) you can then pump the pedal a couple of times before losing all assistance.

Doubt this is you problem though it does sound like a vacum issue. Make sure the hoses to the booster are not collapsing or a blocked.